In oscillators for frequencies on the order of microwave and above, there is an ever increasing need for greater output power. Further, in may instances it is desirable to provide the capability of tuning the oscillator through a given range. In general, voltage controlled oscillators are tuned by means of semiconductor devices, such as varactor diodes and the like. However, because of the high power output requirements of oscillators it is difficult to incorporate varactor tuning because of the high power constraints on the varactor diode. Even if a varactor diode can be obtained which will withstand the power requirements of the oscillator, it wil be extremely expensive. Further, the varactor diode is generally introduced into the circuit where it controls the frequency and power of the fundamental frequency. The high breakdown voltage varactor diodes which must be utilized have a low Q which results in substantial reduction in oscillator efficiency.
In a paper entitled "Second Harmonic Tuning Effects on IMPATT Diode Oscillator Noise Characteristics", by F. J. Sullivan and W. H. From, 1971 G-MTT Symposium Digest, pp. 92-93, a method of controlling the conventionally noisy FM characteristics of an IMPATT diode oscillator by properly terminating the second harmonic frequency is described. These authors discuss the fact that second harmonic circuit tuning in an X band IMPATT diode oscillator can be accomplished. However, this tuning is accomplished by varying the position of a wave guide variable short (mechanically) in which no power problems exist. FUrther, there is no provision for problems of loading the fundamental frequency with the harmonic circuit or discontinuities produced in the circuitry.
A high power microwave oscillator of the type described above is disclosed in U.S. Pat. No. 4,034,314, entitled "Microwave Diode Coaxial Circuit Oscillator Improvement" issued July 5, 1977 and assigned to the same assignee. This patent discloses a high power microwave oscillator with no provision for tuning the oscillator but a second harmonic resonant cavity is disclosed for controlling second harmonic loading of the diode and for reducing noise in the output signal.